This invention relates to dc-dc power converters and, more particularly, to half-bridge, pulse-width modulated dc-to-dc converters.
The conventional half-bridge (HB) pulse-width modulated (PWM) converter is well known and is frequently used for low-to-medium dc-dc power conversion applications. An exemplary form of such converter is shown in FIG. 1 at 10. Normally, the two power switches S1 and S2 in the HB PWM converter are switched alternately with symmetrical duty cycle control and operate with hard switching. The electronic power switches S1 and S2 may be MOSFET or other well known types of electronically controllable, high speed switches and are serially connected across a source 12 of dc power (battery or rectifier coupled to ac source). A junction intermediate the switches S1, S2 is connected to one terminal of primary winding N.sub.p of a power transformer TR through a series capacitor Cb. Transformer TR has a pair of substantially identical windings NS, and NS.sub.2 having a common terminal at 14 connected to a relatively negative dc output terminal 16. Opposite winding end terminals 18 and 20 are connected through respective diode rectifiers D1 and D2 to one end of an output filter inductor Lf. Another end of inductor Lf connects to a relatively positive output terminal 22. A resistive load R.sub.0 and output filter capacitor C.sub.0 connect in parallel to the dc output terminals. The converter 10 operates by gating switch S1 into conduction for a controlled time interval, gating switch S1 out of conduction and then gating switch S2 into conduction. Power is transferred into the converter while S1 conducts. During S2 conduction, the reactive components allow resonant action to occur so that current continues to the load. Due to the high switching loss and high switching noise problems associated with hard-switching operation of the devices S1, S2, the conventional HB PWM converter is not suited for high-frequency operation. In order to reduce the switching loss and noise of the converter, several zero voltage switching (ZVS) techniques have been developed.
Tabisz, Lee, and Jovanovic in U.S. Pat. No. 4,860,184 "Half-Bridge Zero-Voltage Switched Multi-Resonant Converter," disclose a HB converter which can achieve ZVS for both the primary switches (S1, S2) and the secondary rectifying diodes (D1, D2), but it requires the use of a large resonant inductor which results in very high circulating energy and low efficiency. Also, the converter relies on wide-range frequency modulation to regulate the output voltage.
Farrington, Jovanovic, and Lee in U.S. Pat. No. 5,325,283 "Novel Zero-Voltage-Switching Family of Isolated Converters," show a HB converter using symmetrical duty cycle and variable frequency control to achieve ZVS and regulate output voltage. The need for wide-range switching frequency modulation significantly penalizes the performance of the converter by increasing the size of the reactive components (capacitors, transformers, and inductors). In addition, the converter requires a large output filter inductor and also a third rectifying diode.
Ninomiya, Matsumoto, Nakahara and Harada ("Ninomiya") in their 1991 IEEE PESC Record article "Static and Dynamic Analysis of ZVS Half-Bridge Converter with PWM Control" disclose a HB converter using magnetizing energy and asymmetrical duty cycle to achieve ZVS. Its major limitation is in requiring the use of a large output filter inductor when it operates at low duty cycles. This significantly increases the size of the converter and also results in a decrease in conversion efficiency. In addition, this converter exhibits an inherent control stability problem at light load which necessitates the use of a large pre-load for very light load or no load operation.
Imbertson in U.S. Pat. No. 5,245,520 "Asymmetrical Duty Cycle Power Converter" discloses a HB converter which uses asymmetrical duty cycle control and a commutating inductor in series with the power transformer to achieve ZVS. Similar to the Ninomiya converter, Imbertson also requires the use of a large output filter inductor when operating at low duty cycles and exhibits an inherent control stability problem at light load. With the help of a series commutating inductor, the switch conduction loss is somewhat reduced, but ZVS can be easily lost as load current decreases.
FIGS. 2A and 2B show the circuit schematic and switching waveforms respectively of the ZVS HB converter disclosed in the Ninomiya article. Two primary switches S1 and S2 are turned on and off complementary with certain dead times (t0-t1 and t2-t3) in between to allow ZVS to occur. The power transformer TR has one primary winding N.sub.P and two identical secondary windings NS1 and NS2. The transformer is designed to have a fairly low magnetizing inductance so that the peak magnetizing current at either switch S1 or S2 turn-off is greater than the reflected output filter inductor Lf current. In this way, ZVS operation can be maintained even at light load. During the S1 OFF period (t1-t2) and ON period (t3-t0'), the voltage, V.sub.B, appearing in front of the output filter inductor L.sub.f is equal to DV.sub.in /(Np/Ns) and (1-D)V.sub.in /(Np/Ns), respectively, where D is the duty cycle of switch S1 in percent on-time and NP and NS are the number of turns in the primary and each secondary winding, respectively. Obviously, when D is close to 50%, V.sub.B is close to the output voltage (Vo) during both S1 OFF and ON periods, and consequently, only a very small output filter inductor (L.sub.f) is needed. Unfortunately, there are several reasons why the converter can not operate at close to 50% duty cycle at nominal input voltage in most applications. The Ninomiya HB converter shown in FIG. 2A has a voltage conversion-ratio characteristic as shown in FIG. 3. It can be seen that the converter has a maximum gain at 50% duty cycle. In order to accommodate certain load or line transient response requirements as well as component tolerances, the converter is typically operated at around 35% duty cycle even if the steady-state input voltage V.sub.IN is fixed. Considering that in most applications the input voltage has a certain variation range, the steady-state duty cycle is often less than 30%. With such a low duty cycle, V.sub.B will be much high than Vo during S1 ON period and much lower than Vo during S1 OFF period. As a result, the converter requires the use of a large L.sub.f to meet adequate efficiency and filtering requirement. This significantly increases the size and cost of the converter and also results in higher power losses.
Another drawback of the Ninomiya converter shown in FIG. 2A is that it has an inherent control stability problem at very light load or no load. As a result, the converter requires the use of either a pre-load resistor which is simple but very lossy or an active pre-load which is very complicated and expensive to implement.
This invention discloses an improved HB converter that overcomes the above-mentioned drawbacks of the prior art HB converters.